Implants such as stents and occlusive coils have been used in patients for a wide variety of reasons. One of the most common “stenting” procedures is carried out in connection with the treatment of atherosclerosis, a disease which results in a narrowing and stenosis of body lumens, such as the coronary arteries. At the site of the narrowing (i.e., the site of a lesion) a balloon is typically dilatated in an angioplasty procedure to open the vessel. A stent is set in apposition to the interior surface of the lumen in order to help maintain an open passageway. This result may be effected by means of a scaffolding support alone or by virtue of the presence of one or more drugs carried by the stent to aide in the prevention of restenosis.
Various stent designs have been developed and used clinically, but self-expandable and balloon-expandable stent systems and their related deployment techniques are now predominant. Examples of self-expandable stents currently in use are the Magic WALLSTENT® stents and Radius stents (Boston Scientific). A commonly used balloon-expandable stent is the Cypher® stent (Cordis Corporation). Additional self-expanding stent background is presented in: “An Overview of Superelastic Stent Design,” Min. Invas Ther & Allied Technol 2002: 9(3/4) 235-246, “A Survey of Stent Designs,” Min. Invas Ther & Allied Technol 2002: 11(4) 137-147, and “Coronary Artery Stents: Design and Biologic Considerations,” Cardiology Special Edition, 2003: 9(2) 9-14, “Clinical and Angiographic Efficacy of a Self-Expanding Stent” Am Heart J 2003: 145(5) 868-874.
Because self-expanding prosthetic devices need not be set over a balloon (as are balloon-expandable designs), self-expanding stent delivery systems can be designed to a relatively smaller outer diameter than their balloon-expandable counterparts. As such, self-expanding stents may be better suited to reach the smallest vasculature or to achieve access in more difficult cases.
One consideration pertinent to the delivery of self-expanding stent designs concerns frictional forces internal to the subject delivery system. Internal forces can be a significant issue with respect to system actuation. Testing by the assignee hereof has clearly demonstrated a loss of motive force available to actuate a distally located restraint when the delivery system is subject to conditions of or simulating tortuous anatomy. As such, systems that require little or no direct user actuation of components to effect stent release are desirable. Examples of such systems are presented in a number of patents.
U.S. Pat. No. 5,873,907 offers a system employing circumferential bands, each having an electrolytically erodable section, where the bands hold a stent in a collapsed configuration until the sections' joints are released. U.S. Pat. No. 5,980,514 discloses a system in which a self-expanding stent is held onto a wire directly by electrolytically erodable joints. U.S. Pat. No. 6,716,238 describes a system in which one or more tethers are connected to a stent to releasably restrain it for delivery or to stabilize its position during delivery. The tethers may work in conjunction with a sheath to hold a distal end of the stent, allowing withdrawal of the sheath from the stent to effect release. When the sheath is withdrawn, the stent expands radially, but is not fully released until the connections between the tethers and the stent are mechanically broken, electrolytically eroded or melted away. In another embodiment described in the '238 patent, one or more tethers is wrapped around the body of the stent to tie it down until released.
Upon closer examination, each of the referenced systems has serious limitations—either in terms of clinical practicality, basic mechanical feasibility or both. Issues variously presented by the referenced stent delivery systems include:
1) risk of tissue damage or vulnerable plaque dislodgement by drawing released tethers or bands past/between an open/opening stent and a vessel wall;
2) inability to employ drug elution matrix upon a stent because of direct connection of an erodable solder joint to the stent;
3) unpredictability of joint or release segment corrosion;
4) long deployment times as a result of large amounts of material to erode to effect implant release; and/or
5) bulkiness in system design.
Other issues may be presented as well as appreciated by those with skill in the art. Accordingly, to maximize the potential benefits of electrolytic release for stents and/or other implants, there exists a need for improved systems.